Journal of Catalysis 211, 165–172 (2002) doi:10.1006/jcat.2002.3726 n-Butene Conversion on H-Ferrierite Studied by 13 C MAS NMR Alexander G. Stepanov, ∗,1 Mikhail V. Luzgin, ∗ Sergei S. Arzumanov, ∗ Horst Ernst,† and Dieter Freude† ,2 ∗ Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia; and †Abteilung Grenzfl ¨ achenphysik, Universit¨ at Leipzig, Linn´ estraße 5, 04103 Leipzig, Germany Received March 5, 2002; revised June 6, 2002; accepted June 19, 2002 13 C MAS NMR analysis of the hydrocarbon products formed from the selectively 13 C-labeled n-but-1-ene on zeolite ferrierite (H- FER) in a closed batch reactor revealed the following successive steps of the olefin conversion with temperature increase from 300 to 823 K: a double-bond-shift reaction, scrambling of the selective 13 C label in the formed n-but-2-ene, oligomerization (dimerization), conjunct polymerization, formation of condensed aromatics, and formation of the simple aromatics. Arguments in favor of either bimolecular or pseudo-monomolecular mechanisms are provided, excluding at the same time the monomolecular isomerization of n- to isobutene on a fresh sample. The arguments are based on selective label redistribution in the n-but-2-enes, the impossibility of the existence of isobutene inside the pores of the zeolite under static conditions and the observation of n-but-2-enes oligomeriza- tion (dimerization). Conjunct polymerization leads to the forma- tion of alkyl-substituted cyclopentenyl cations (CPCs), which can serve as an intermediate for pseudo-monomolecular isomerization. Carbonaceous deposits (polycyclic aromatics), which deactivate the catalyst in the isomerization reaction, are formed from the CPCs. Polycyclic aromatics are transformed into simple aromatics with methane and ethane evolution at 823 K. c  2002 Elsevier Science (USA) Key Words: n-butene; H-FER zeolite; isomerization; conjunct polymerization; 13 C-label scrambling; reaction mechanism; 13 C MAS NMR spectroscopy. 1. INTRODUCTION Zeolite H-FER, the hydrogen form of ferrierite (1), represents a high-efficiency catalyst for isomerization of n-butene into isobutene (2, 3). Industrial demands for isobutene are due to its use in the synthesis of methyl tert- butyl ether (MTBE), an important component of the gaso- line pool. High selectivity and stability of the ferrierite in the isomerization of n- to isobutene (2, 3) stimulated nu- merous studies of the mechanism of this reaction (3–17). Monomolecular, bimolecular, and pseudo-monomolecular 1 To whom correspondence should be addressed. Fax: +49 341 97 32549. E-mail: freude@physik.uni-leipzig.de. 2 To whom correspondence should be addressed. Fax: +7 (3832) 34 30 56. E-mail: a.g.stepanov@catalysis.nsk.su. mechanisms have been claimed for this reaction by studying the products distribution (3), the changes of the selectivity in the products with time on stream (5, 6, 9) and the 13 C isotopic label redistribution (6, 12) from the reagent into the reaction products. A bimolecular reaction pathway with simultaneous for- mation of a large quantity of C 3 and C 5 olefins was first suggested by Mooiweer et al. (3). Meriaudeau et al. (6) pro- posed that a bimolecular mechanism operates on the fresh and nonselective ferrierite catalyst, whereas a monomolec- ular reaction takes place for the coked and selective cata- lyst. The conclusion about a bimolecular mechanism at the outer surface of the catalyst was made based on the poor selectivity for isobutene formation and observation of the double 13 C-labeled isobutene from the single 13 C- labeled n-butene in the reaction products (6–8). The in- creased selectivity of isobutene formation in parallel with the decreased total activity of the coked catalysts and the yield of the single 13 C-labeled isobutene from the single labeled n-butene were the main arguments in favor of a monomolecular isomerization mechanism inside the zeo- lite channels (4, 6, 8). The monomolecular mechanism in- cludes the formation of a high-energy primary carbenium ion (18, 19). A pseudo-monomolecular mechanism was as- sumed by Guisnet et al. (9–11). It excludes the formation of a primary cation and involves the participation of the coke deposits generating intermediate benzylic cations as the ac- tive sites near the outer surface of the crystallites. But the formation of benzylic cations was so far not experimentally proved and other cationic species may be involved in the reaction. Various spectroscopic techniques were used to charac- terize the adsorbed species and clarify the reaction mecha- nism (6–16, 20, 21). UV-vis and FTIR spectroscopic studies characterized the adsorbed species, precursors of isobutene and high-temperature coke deposits (11–13, 15, 16, 20, 21), whereas 13 C NMR and GC-MS monitored the 13 C label redistribution from n-butene to isobutene showing the dif- ference in the reaction mechanisms on fresh and coked cata- lysts (6, 12). 13 C MAS NMR spectroscopy is a well-established tech- nique for the study of the mechanisms of heterogeneous 165 0021-9517/02 $35.00 c  2002 Elsevier Science (USA) All rights reserved.